This isnt sudden, nor inexplicable. He hasn't been good since 2016.---(Talking about Roark)
Moreover, if the Nats expect to compete for the division going forward, Rizzo needs to get another top of the rotation starter. Now is as good a time as any
ERAs as of June 1 except Roark's on May 26.
Roark's ERA for his first 10 games was 3.17
Gio's ERA for his first 11 games was 2.10
Scherzer's ERA his first 12 starts was 1.92
Strasburg's ERA his first 11 games was 3.12
All 4 pitchers were also in the top 4 starters in pitches per start at that point. Martinez over extending his starters has created a lot of unnecessary problems and has maybe cost the division title.
Back to your point, I agree they do need another starter. And a catcher and a short and a long reliever.
I don't agree that Roark wasn't good earlier this year.